Non-aqueous electrolyte and lithium secondary battery using the same

ABSTRACT

The present invention provides non-aqueous electrolyte solution for a lithium secondary battery, comprising fluoroethylene carbonate and a pyrimidine-based compound; and a lithium secondary battery using the same.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No. PCT/KR2012/005731 filed on Jul. 18, 2012, which claims priority under 35 USC 119(a) to Korean Patent Application No. 10-2011-0071048 filed in the Republic of Korea on Jul. 18, 2011, and Korean Patent Application No. 10-2012-0078055 filed in the Republic of Korea on Jul. 18, 2012, the disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery, which comprises fluoroethylene carbonate and a pyrimidine-based compound, and a lithium secondary battery using the same.

BACKGROUND ART

Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for high energy density of batteries used as a source of power supply of such devices is increasing. Therefore, research and development of lithium secondary batteries, which most meet the demand, are actively being conducted.

Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's comprises an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.

The lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. In order to provide such a high operation voltage, an electrolyte composition electrochemically stable in a charging/discharging voltage range of 0 to 4.2 V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate and propylene carbonate, and a linear carbonate compound such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate are suitably mixed to be used as a solvent of the electrolyte solution. A typical electrolyte solution uses lithium salt such as LiPF₆, LiBF₄ and LiClO₄ as solutes, which acts as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.

During the initial charging process of the lithium secondary battery, lithium ions emitted from a cathode active material such as a lithium-metal oxide is transferred into an anode active material such as graphite and inserted between the layers of the anode active material, at which high reactive lithium reacts with the electrolyte solution and carbon present in the anode active material on the surface of the anode active material such as graphite to produce a compound such as Li₂CO₃, Li₂O and LiOH. The produced compound forms a kind of a solid electrolyte interface (SEI) layer on the surface of the anode active material such as graphite.

The SEI layer functions as an ion tunnel, allowing only lithium ions to pass through. As an effect of such an ion tunnel, the SEI layer prevents the molecule of an organic solvent having large molecular weight, which is included in the electrolyte solution and transferred together with lithium ions, from being inserted between the layers of the anode active material to destroy the structure of the anode. As a result, direct contact of the electrolyte solution with the anode active material is prohibited to prevent the decomposition of the electrolyte solution and reversibly maintains the amount of lithium ions present in the electrolyte solution, thereby allowing a stable charging/discharging.

However, during the formation of the SEI layer, the carbonate-based organic solvent may decompose to generate a gas such as CO, CO₂, CH₄ and C₂H₆, which may cause the battery being charged to swell in thickness. Also, when a battery is left at a high temperature in a fully charged state, the SEI layer may be slowly broken down due to increased electrochemical energy and thermal energy over time, thereby causing continuous side reactions between the surface of the anode and the surrounding electrolyte solution and continuously generating gas. As a result, the inner pressure of the battery may be increased, thereby increasing the thickness of the battery to cause performance problems to electronics such as cellular phones and notebook computers equipped with the battery. Thus, the SEI layer has poor stability at a high temperature. In addition, in conventional lithium secondary batteries comprising a large amount of ethylene carbonate, the unstableness of the SEI layer may intensify the inner pressure increase problem of the battery. Furthermore, ethylene carbonate has a high freezing point (37 to 39° C.) and maintains a solid state at room temperature to have low ionic conductivity at a low temperature. Accordingly, a lithium battery using a non-aqueous solvent containing a large amount of ethylene carbonate has a poor conductivity at a low temperature.

In order to overcome such a problem, attempt has been made to vary the composition of a carbonate-based organic solvent or mix the solvent with a certain additive to change a SEI layer-forming reaction. However, it is known that the variation of a solvent composition or the addition of a certain compound may improve some performances of a battery, but may also deteriorate any other properties thereof.

Accordingly, there is a need to develop a composition of a non-aqueous electrolyte solution capable of providing a lithium battery having superior characteristics in terms of life cycle, low and high temperature discharging, as well as high-rate charging/discharging characteristic.

DISCLOSURE Technical Problem

The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a non-aqueous electrolyte solution for a lithium secondary battery having an improved life cycle at room temperature and a high temperature, and a lithium secondary battery comprising the non-aqueous electrolyte solution.

Technical Solution

In order to accomplish the above object, in accordance with one aspect of the present invention, there is provided a non-aqueous electrolyte solution for a lithium secondary battery, comprising an electrolyte salt and an organic solvent, wherein the non-aqueous electrolyte solution further comprises fluoroethylene carbonate and a pyrimidine-based compound of the following formula (I), and the amount of the fluoroethylene carbonate is in the range of 0.1 to 50 vol % based on the total volume of the organic solvent and the fluoroethylene carbonate:

-   -   wherein, R₁ to R₄ are each independently hydrogen, substituted         or unsubstituted alkyl group having 1 to 10 carbon atoms, or         substituted or unsubstituted alkoxy group having 1 to 10 carbon         atoms.

In accordance with another aspect of the present invention, there is provided a lithium secondary battery comprising an electrode assembly consisting of a cathode, an anode and a separator interposed therebetween, and a non-aqueous electrolyte solution introduced in the electrode assembly, wherein the non-aqueous electrolyte solution is the above non-aqueous electrolyte solution for a lithium secondary battery.

Advantageous Effects

The non-aqueous electrolyte solution according to one aspect of the present invention, which comprises fluoroethylene carbonate and a pyrimidine-based compound, is used in a lithium secondary battery to minimize the capacity decrease and provide a markedly improved life characteristics and stability even if charging/discharging cycles are repeated several hundred times at room temperature and a high temperature.

DESCRIPTION OF DRAWINGS

Other objects and aspects of the present disclosure will become apparent from the following descriptions of the embodiments with reference to the accompanying drawings in which:

FIG. 1 shows dQ/dV curves over initial charging voltages of batteries wherein the amount of pyrimidine used in each battery is varied.

FIG. 2 shows the high-temperature life characteristics of lithium batteries prepared in Examples 2-1 to 2-4 and Comparative Example 2-1.

FIG. 3 shows the high-temperature life characteristics of lithium batteries prepared in Examples 2-5 to 2-9 and Comparative Example 2-1.

FIG. 4 shows the high-temperature life characteristics of lithium batteries prepared in Example 2-1 and Comparative Examples 2-2 to 2-4.

FIG. 5 shows the high-temperature life characteristics of lithium batteries prepared in Examples 2-1, 2-10 and 2-11 and Comparative Examples 2-5 and 2-6.

BEST MODE

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation.

The non-aqueous electrolyte solution for a lithium secondary battery according to one aspect of the present invention comprises an electrolyte salt and an organic solvent, and further comprises fluoroethylene carbonate and a pyrimidine-based compound of the following formula (I), and the amount of the fluoroethylene carbonate is in the range of 0.1 to 50 vol % based on the total volume of the organic solvent and the fluoroethylene carbonate:

-   -   wherein, R₁ to R₄ are each independently hydrogen, substituted         or unsubstituted alkyl group having 1 to 10 carbon atoms, or         substituted or unsubstituted alkoxy group having 1 to 10 carbon         atoms.

The above-mentioned substituents have specific meanings as follows:

The term “alkyl group” used herein refers to a straight- or branched-chain saturated monohydric hydrocarbon moiety having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms. Examples of unsubstituted alkyl group include methyl, ethyl, propyl, hexyl and the like. In the alkyl group, at least one hydrogen contained therein may be optionally substituted with halogen, hydroxyl, —SH, nitro, cyano, substituted or unsubstituted amino (such as —NH₂, —NH(R) and —N(R′)(R″), wherein R′ and R″ are each independently C₁₋₁₀ alkyl), amidino, hydrazine, hydrazone, carboxyl, sulfonic acid, phosphoric acid, C₁₋₂₀ alkyl, halogenated C₁₋₂₀ alkyl, C₁₋₂₀ alkenyl, C₁₋₂₀ alkynyl, C₁₋₂₀ heteroalkyl, C₆₋₂₀ aryl, C₆₋₂₀ arylalkyl, C₆₋₂₀ heteroaryl, or C₆₋₂₀ heteroarylalkyl.

The term “alkoxy group” used herein refers to a straight- or branched-chain saturated oxygen-containing hydrocarbon moiety having 1 to 10 carbon atoms, preferably 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms. Examples of unsubstituted alkoxy group include methoxy, ethoxy, propoxy, butoxy and t-butoxy. The alkoxy group may further be substituted with at least one halogen atom such as F, Cl and Br to provide haloalkoxy group, for example, fluoromethoxy, chloromethoxy, trifluoromethoxy, trifluoroethoxy, fluoroethoxy and fluoropropoxy. In the alkoxy group, at least one hydrogen may be optionally substituted with substituents as mentioned in the definition of the alkyl group.

The fluoroethylene carbonate comprises fluorine having a strong electron-attracting function, thereby allowing the formation of an SEI layer having a high dielectric rate in the initial charging of a battery, good lithium-ion conductivity and good durability. Accordingly, when the fluoroethylene carbonate is added to a non-aqueous electrolyte solution as an additive, a secondary battery using such a non-aqueous electrolyte solution can have high-rate life characteristics and high-capacity charging characteristics per unit of time.

In a secondary battery in which a non-aqueous electrolyte solution is introduced, an anode is subject to rapid shrinking-swelling as the charging/discharging cycles of the battery are repeated, at which an SEI layer is destroyed due to the anode swelling during charging, and the electrolyte solution decomposes to form a new SEI layer. From this phenomenon, the electrolyte is gradually depleted to exhaust lithium ions present in the electrolyte solution as the cycles proceed, thereby decreasing the capacity of the battery.

Accordingly, the non-aqueous electrolyte solution according to one aspect of the present invention comprises a pyrimidine-based compound of formula (I) so as to minimize the capacity decrease of a battery and remarkably improve life characteristics. Specifically, the pyrimidine-based compound can react with an anode first prior to fluoroethylene carbonate as well as a conventional carbonate-based solvent used in an electrolyte solution, to form a stable and dense film on the surface of the anode, thereby inhibiting the continuous decomposition reaction between fluoroethylene carbonate and the anode. Accordingly, the pyrimidine-based compound can be used together with fluoroethylene carbonate to improve battery characteristics, as compared to the use of only fluoroethylene carbonate.

That is, the pyrimidine-based compound forms a polymerizable film on the surface of an anode, and due to the nitrogen atom of the polymerizable film having high electron-density, the polymerizable film exhibits good trapping of a metal cation. Accordingly, a metal cation having low electron-density can be trapped between the polymerizable film and a film on an electrode which is in contact with the polymerizable film. As a result, this allows to inhibit a metal cation generated from a cathode active material layer from leaching as a metal on the surface of the anode.

Thus, from the trapping-effect of a metal cation of the pyrimidine-based compound, discharge rate characteristics under the condition of high-temperature preservation is maintained, thereby improving the high-temperature preservation of a secondary battery.

Also, if an electrolyte solution is left for a period after it is prepared, the amount of moisture and HF therein increases to reduce the discharging capacity of a battery. However, the use of the pyrimidine-based compound can inhibit the variation of the amount of moisture and HF by the reaction of a nitrogen atom present in the pyrimidine-based compound with HF, thereby preventing the discharging capacity of a battery from being reduced.

Meanwhile, referring to FIG. 1, it can be confirmed that the pyrimidine-based compound first reacts with an anode prior to fluoroethylene carbonate to form a stable film on the surface of the anode. For reference, each secondary battery to be tested was prepared by obtaining electrodes of a cathode made of LiCoO₂ and an anode made of nature graphite; and introducing in the electrodes each non-aqueous electrolyte solution obtained by mixing fluoroethylene carbonate (FEC), propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a ratio of 3:1:6 (vol %) as an organic solvent, adding thereto each pyrimidine (PYM) in an amount of 0, 0.5, 1, 2 or 3 parts by weight, and 1,3-propane sultone in an amount of 2 parts by weight, based on 100 parts by weight of the organic solvent, and dissolving LiPF₆ therein until 1M LiPF₆ solution was obtained.

Specifically, FIG. 1 shows dQ/dV curves over initial charging voltages of each battery wherein a different amount of a pyrimidine (PYM) is each used relative to a certain amount of fluoroethylene carbonate. As the amount of the pyrimidine increases, the height of peaks increases at an area around 1.8V while the height of peaks decreases at an area around 2.4V. The peaks at an area around 1.8V are due to the pyrimidine, which means that the pyrimidine first forms a stable film prior to fluoroethylene carbonate in an anode.

The amount of the fluoroethylene carbonate is in the range of 0.1 to 50 vol %, preferably 1 to 40 vol %, more preferably 3 to 30 vol %, based on the total volume of the organic solvent and the fluoroethylene carbonate. When the amount of the fluoroethylene carbonate satisfies the above range, the fluoroethylene carbonate may be maintained in a constant amount without the exhaustion thereof for a long-term cycle progress, a rise on the cost of a battery may be controlled, battery performances may be prevented from being deteriorated during a high-rate discharging due to an excessive increase in the resistance of a cathode, and the electrolyte solution may be uniformly penetrated into electrodes after being introduced.

The amount of the pyrimidine-based compound is in the range of 0.01 to 20 parts by weight, preferably 0.1 to 10 parts by weight, more preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the sum of the organic solvent and the fluoroethylene carbonate. When the amount of the pyrimidine-based compound satisfies the above range, high-temperature performances may be sufficiently improved and the viscosity of the electrolyte solution may not excessively increase, thereby improving performances at room temperature and a high temperature, and battery performances may be prevented from being deteriorated during a high-rate discharging due to an excessive increase in the resistance of a cathode.

The pyrimidine-based compound may be, for example, at least one selected from the group consisting of pyrimidine, 2-methyl pyrimidine, 4-methyl pyrimidine, 5-methyl pyrimidine, 4,6-dimethyl pyrimidine, 4,6-dimethoxy pyrimidine, 2-ethyl pyrimidine, 4-ethyl pyrimidine, 5-ethyl pyrimidine, 4,6-diethyl pyrimidine, 2-methoxy pyrimidine, 4-methoxy pyrimidine, 5-methoxy pyrimidine, 2-ethoxy pyrimidine, 4-ethoxy pyrimidine and 5-ethoxy pyrimidine, but is not limited thereto.

The non-aqueous electrolyte solution according to one aspect of the present invention comprises a lithium salt as an electrolyte salt. The lithium salt may be any one which is conventionally used in an electrolyte solution for a lithium secondary battery. For example, an anion of the lithium salt may be any one selected from the group consisting of F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻ and (CF₃CF₂SO₂)₂N⁻.

The non-aqueous electrolyte solution comprises an organic solvent which is conventionally used in an electrolyte solution for a lithium secondary battery, for example, an ether, an ester, an amide, a linear carbonate, a cyclic carbonate or a mixture thereof.

Among these, a linear carbonate, a cyclic carbonate, or a mixture thereof is representatively used.

The cyclic carbonate compound may be selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinyl ethylene carbonate, a halide thereof and a mixture thereof.

The linear carbonate compound may be any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate and a mixture thereof, but is not limited thereto.

In particular, among the above carbonate-based organic solvents, the cyclic carbonates such as ethylene carbonate and propylene carbonate have a high viscosity and a high dielectric constant to more easily dissociate a lithium salt in an electrolyte. Such a cyclic carbonate may be mixed with a linear carbonate with low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate in a suitable ratio to provide an electrolyte solution with a high electric conductivity.

Also, the ether which may be used as the organic solvent is any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether and a mixture thereof, but is not limited thereto.

In addition, the ester which may be used as the organic solvent is any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, ε-caprolactone and a mixture thereof, but is not limited thereto.

The non-aqueous electrolyte solution for a lithium secondary battery according to one aspect of the present invention may further comprise an SEI layer-forming additive within a range which does not deviate from the object of the present invention. The SEI layer-forming additive which may be used in the present invention includes a cyclic sulfite, a saturated sultone, an unsaturated sultone, a non-cyclic sulfone and a mixture thereof, but is not limited thereto. Among the cyclic carbonates mentioned above, vinylene carbonate and vinyl ethylene carbonate may also be used as the SEI layer-forming additive to improve the life of a battery.

Examples of the cyclic sulfite include ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethyl propylene sulfite, 4,5-diethyl propylene sulfite, 4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite and 1,3-butylene glycol sulfite; examples of the saturated sultone include 1,3-propane sultone and 1,4-butane sultone; examples of the unsaturated sultone include ethene sultone, 1,3-propene sultone, 1,4-butene sultone and 1-methyl-1,3-propene sultone; and examples of the non-cyclic sulfone include divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone and methyl vinyl sulfone.

The SEI layer-forming additive may be used in a suitable amount depending on its specific type, for example, in an amount of 0.01 to 10 parts by weights based on 100 parts by weight of the non-aqueous electrolyte solution.

Even if the pyrimidine-based compound forms a stable film on the anode, a solid may be formed by the addition of the additives to lower a performance-improvement effect due to the formation of the stable film.

In this regard, among the additives as mentioned above, the cyclic sulfite or unsaturated sultone, in particular, ethylene sulfite or 1,3-propene sultone is preferably used since they have a low reactivity with the pyrimidine-based compound to not form a solid, thereby being capable of increasing the above-mentioned synergetic effect due to the combination of the fluoroethylene carbonate and the pyrimidine-based compound.

The non-aqueous electrolyte solution may be instantly used as a liquid electrolyte or immersed in a polymer and then used in the form of a gel polymer electrolyte in a lithium secondary battery.

The non-aqueous electrolyte solution according to one aspect of the present invention may be prepared by mixing the electrolyte salt with the non-aqueous solvent and fluoroethylene carbonate, and dissolving a pyrimidine-based compound of formula (I) therein.

At this time, the compounds being added to the electrolyte solution and the non-aqueous solvent are subject to pre-purification before using within a range which will not substantially deteriorate productivity, so as to have little or no impurity.

In the non-aqueous electrolyte solution, air or carbon dioxide may be added to improve battery characteristics including the inhibition of gas generation by electrolyte solution decomposition, cycle characteristics over a long period and charging-preservation characteristics.

In order to improve charging/discharging characteristics, carbon dioxide may be used by dissolving in the non-aqueous electrolyte solution. The carbon dioxide may be dissolved in an amount of 0.001 wt % or more, preferably 0.05 wt % or more, more preferably 0.2 wt % or more, most preferably until the carbon dioxide is saturated in the non-aqueous electrolyte solution.

Further, in accordance with the present invention, there is provided a lithium secondary battery comprising an electrode assembly consisting of a cathode, an anode and a separator interposed therebetween, and a non-aqueous electrolyte solution introduced in the electrode assembly, wherein the non-aqueous electrolyte solution is the above-mentioned non-aqueous electrolyte solution for a lithium secondary battery.

The cathode, anode and separator composing the electrode assembly may be any one which is conventionally used in the preparation of a lithium secondary battery.

The cathode has a structure wherein a cathode layer comprising a cathode active material, a conductive material and a binder is immersed in one side or both sides of a current collector.

As the cathode active material, a lithium-containing transition metal oxide may be preferably used, for example, any one selected from the group consisting of Li_(x)CoO₂(0.5<x<1.3), Li_(x)NiO₂(0.5<x<1.3), Li_(x)MnO₂(0.5<x<1.3), Li_(x)Mn₂O₄(0.5<x<1.3), Li_(x)(Ni_(a)Co_(b)Mn_(c))O₂(0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1), Li_(x)Ni_(1-y)Co_(y)O₂(0.5<x<1.3, 0<y<1), Li_(x)Co_(1-y)Mn_(y)O₂(0.5<x<1.3, 0≦y<1), Li_(x)Ni_(1-y)Mn_(y)O₂(0.5<x<1.3, O≦y<1), Li_(x)(Ni_(a)Co_(b)Mn_(c))O₄(0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2), Li_(x)Mn_(2-z)Ni_(z)O₄(0.5<x<1.3, 0<z<2), Li_(x)Mn_(2-z)Co_(z)O₄(0.5<x<1.3, 0<z<2), Li_(x)CoPO₄(0.5<x<1.3), LixFePO₄(0.5<x<1.3) and a mixture thereof may be used.

The lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) and a metal oxide. In addition, lithium-containing transition metal sulfide, selenide, or halide may also be used.

As the conductive material, any electron-conductive material is not particularly limited if it does not cause a chemical change in an electrochemical device. Generally, carbon black, graphite, carbon fiber, carbon nanotube, metal powders, a conductive metal oxide and an organic conductive material may be used. Examples of a commercially available conductive material include acetylene black-based products (Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC-based products (Armak Company, Vulcan XC-72 (Cabot Company) and Super P (MMM Carbon Company). For example, acetylene black, carbon black and graphite may be used.

The anode has a structure wherein an anode layer comprising an anode active material and a binder is immersed in one side or both sides of a current collector.

As the anode active material, a carbon-based material, lithium metal, a metal compound or a mixture thereof which can conventionally intercalate and disintercalate lithium ions may be used.

Specifically, the carbon-based material may be low-crystalline carbon or high-crystalline carbon. Representative examples of the low-crystalline carbon include soft carbon and hard carbon, and representative examples of the high-crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature sintered carbon such as petroleum or coal tar pitch derived cokes.

The metal compound may be a compound containing at least one metal selected from Si, Ge, Sn, Pb, P, Sb, Bi, Al, Ga, In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, Mg, Sr and Ba, or a mixture thereof. Such a metal compound may be used in the form of a combination, an alloy, an oxide (e.g., TiO₂ and SnO₂), a nitride, a sulfide, a boride, an alloy with lithium and any other form, and among these, forms such as a combination, an alloy, an oxide and an alloy with lithium can provide the high capacity of a battery. In particular, a compound containing at least one metal selected from Si, Ge and Sn, preferably Si and Sn can provide even higher capacity of a battery.

The binder used in the cathode and anode functions to maintain cathode and anode active materials in a current collector and connect active materials with each other, and may be any one which is conventionally used.

For example, various kinds of binder polymers including vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) may be used.

The current collector used in the cathode and anode may be made of any high conductive metal as long as a slurry of the active material can easily adhere and has no reactivity within the voltage range of a battery. Specifically, non-limiting examples of a cathode current collector include aluminum foils, nickel foils and a combination thereof, and non-limiting examples of an anode current collector include copper foils, gold foils, nickel foils, copper alloy foils and a combination thereof. The current collector may be used in a laminated form of substrates made of such materials.

Each of the cathode and anode may be prepared by mixing an active material, a binder and a solvent having a high boiling point to form an electrode composite and applying the composite on the copper foil of a current collector, followed by drying, pressing and then heat-treatment at a temperature of 50 to 250° C. under vacuum for 2 hours.

Also, the cathode has a layer thickness (per one side of a current collector) of 30 to 120 μm, preferably 50 to 100 μm, and the anode has a layer thickness of 1 to 100 μm, preferably 3 to 70 μm. When the cathode and the anode satisfies such a thickness range, a sufficient amount of an active material is provided in the layer of an electrode material to prevent a battery capacity from being lowered and improve cycle and rate characteristics.

The separator may be obtained from a porous polymer film which is conventionally used alone or in the form of lamination in conventional separators, for example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer and ethylene/methacrylate copolymer. Also, as the separator, conventional porous non-woven fabrics such as a non-woven fabric made of glass fiber having a high melt point or polyethylene terephthalate fiber may be used, but is not limited thereto.

The lithium secondary battery of the present invention is not limited to its shape, but may be a cylindrical shape using a can, an angled shape, a pouch shape or a coin shape.

Hereinafter, the present invention will be explained in more detail with reference to the following Examples. However, it should be understood that the Examples are provided for the purpose of illustrations only and to better explain to a person having ordinary skill in the art, and is not intended to limit the scope of the present invention, so other equivalents and modifications could be made thereto without departing from the spirit and scope of the present invention.

Preparation of Non-Aqueous Electrolyte Solution

EXAMPLE 1-1

Fluoroethylene carbonate (FEC), propylene carbonate (PC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:10:60 (vol %) to obtain a solvent mixture. Thereto, 1 part by weight of pyrimidine and 2 parts by weight of 1,3-propane sultone were added based on 100 parts by weight of the mixture, and LiPF₆ was dissolved therein until 1M LiPF₆ solution was obtained, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-2

The procedure of Example 1-1 was repeated except that ethylene sulfite was used instead of 1,3-propane sultone, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-3

The procedure of Example 1-1 was repeated except that 2-methyl pyrimidine was used instead of pyrimidine, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-4

The procedure of Example 1-1 was repeated except that 5-methyl pyrimidine was used instead of pyrimidine, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-5

The procedure of Example 1-1 was repeated except that pyrimidine is used in an amount of 2 parts by weight, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-6

The procedure of Example 1-1 was repeated except that 2 parts by weight of 4,6-dimethyl pyrimidine was used instead of 1 part by weight of pyrimidine, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-7

The procedure of Example 1-1 was repeated except that 2 parts by weight of 4,6-dimethoxy pyrimidine was used instead of 1 part by weight of pyrimidine, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-8

The procedure of Example 1-1 was repeated except that 2 parts by weight of 4-methyl pyrimidine was used instead of 1 part by weight of pyrimidine, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-9

The procedure of Example 1-1 was repeated except that ethylene sulfite was used instead of 1,3-propane sulfone, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-10

Fluoroethylene carbonate (FEC), propylene carbonate (PC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 10:15:75 (vol %) to obtain a solvent mixture. Thereto, 1 part by weight of pyrimidine and 2 parts by weight of 1,3-propane sultone were added based on 100 parts by weight of the mixture, and LiPF₆ was dissolved therein until 1M LiPF₆ solution was obtained, to prepare a non-aqueous electrolyte solution.

EXAMPLE 1-11

The procedure of Example 1-10 was repeated except that pyrimidine was used in an amount of 10 parts by weight, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-1

The procedure of Example 1-1 was repeated except that pyrimidine was not used, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-2

The procedure of Example 1-1 was repeated except that pyrazine was used instead of pyrimidine, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-3

The procedure of Example 1-1 was repeated except that pyridazine was used instead of pyrimidine, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-4

The procedure of Example 1-1 was repeated except that imidazole was used instead of pyrimidine, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-5

The procedure of Example 1-10 was repeated except that pyrimidine was not used, to prepare a non-aqueous electrolyte solution.

Comparative Example 1-6

Fluoroethylene carbonate (FEC), propylene carbonate (PC) and ethyl methyl carbonate were mixed in a volume ratio of 60:5:35 (vol %) to obtain a solvent mixture. Thereto, 1 part by weight of pyrimidine and 2 parts by weight of 1,3-propane sultone were added based on 100 parts by weight of the mixture, and LiPF₆ was dissolved therein until 1M LiPF₆ solution was obtained, to prepare a non-aqueous electrolyte solution.

Preparation of Lithium Secondary Battery

EXAMPLE 2-1

LiCoO₂ as a cathode and natural graphite as an anode were used to obtain electrodes, in which the non-aqueous electrolyte solution prepared in Example 1-1 was introduced, according to a conventional method known in the art, to prepare a lithium secondary battery.

EXAMPLE 2-2

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-2 was used, to prepare a lithium secondary battery.

EXAMPLE 2-3

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-3 was used, to prepare a lithium secondary battery.

EXAMPLE 2-4

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-4 was used, to prepare a lithium secondary battery.

EXAMPLE 2-5

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-5 was used, to prepare a lithium secondary battery.

EXAMPLE 2-6

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-6 was used, to prepare a lithium secondary battery.

EXAMPLE 2-7

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-7 was used, to prepare a lithium secondary battery.

EXAMPLE 2-8

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-8 was used, to prepare a lithium secondary battery.

EXAMPLE 2-9

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-9 was used, to prepare a lithium secondary battery.

EXAMPLE 2-10

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-10 was used, to prepare a lithium secondary battery.

EXAMPLE 2-11

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Example 1-11 was used, to prepare a lithium secondary battery.

Comparative Example 2-1

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-1 was used, to prepare a lithium secondary battery.

Comparative Example 2-2

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-2 was used, to prepare a lithium secondary battery.

Comparative Example 2-3

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-3 was used, to prepare a lithium secondary battery.

Comparative Example 2-4

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-4 was used, to prepare a lithium secondary battery.

Comparative Example 2-5

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-5 was used, to prepare a lithium secondary battery.

Comparative Example 2-6

The procedure of Example 2-1 was repeated except that the non-aqueous electrolyte solution prepared in Comparative Example 1-6 was used, to prepare a lithium secondary battery.

Evaluation of Lithium Secondary Battery Characteristics

Life Characteristics at a High Temperature (1)

Lithium secondary batteries (Capacity: 5.5 mAh) prepared in Examples 2-1 to 2-4 and Comparative Example 2-1 were charged at 60° C. with a constant current of 1.0 C up to 4.35 V and then with a constant voltage of 4.35 V, and the charging process was completed when the charging current reached 0.275 mA. After left for 10 minutes, the batteries were discharged with a constant current of 1.0 C up to 3.0 V. The charging/discharging was repeated 50 times under the same conditions, and then the capacity of the batteries were measured and shown in FIG. 2. The unit “C” which is used herein refers to a charging/discharging current rate (C-rate) of a battery, represented by ampere (A), and is usually expressed by a percentage of the battery capacity. Accordingly, “1 C” of the batteries prepared above means a current of 5.5 mA.

Life Characteristics at a High Temperature (2)

Lithium secondary batteries (Capacity: 5.5 mAh) prepared in Examples 2-5 to 2-9 and Comparative Example 2-1 were charged at 60° C. with a constant current of 1.0 C up to 4.35 VA and then with a constant voltage of 4.35 V, and the charging process was completed when the charging current reached 0.275 mA. After left for 10 minutes, the batteries were discharged with a constant current of 1.0 C up to 3.0 V. The charging/discharging was repeated 100 times under the same conditions, and then the capacity of the batteries were measured and shown in FIG. 3. The unit “C” which is used herein refers to a charging/discharging current rate (C-rate) of a battery, represented by ampere (A), and is usually expressed by a percentage of the battery capacity. Accordingly, “1 C” of the batteries prepared above means a current of 5.5 mA.

Life Characteristics at a High Temperature (3)

Lithium secondary batteries (Capacity: 5.5 mAh) prepared in Example 2-1 and Comparative Examples 2-2 to 2-4 were charged at 60° C. with a constant current of 1.0 C up to 4.35 VA and then with a constant voltage of 4.35 V, and the charging process was completed when the charging current reached 0.275 mA. After left for 10 minutes, the batteries were discharged with a constant current of 1.0 C up to 3.0 V. The charging/discharging was repeated 100 times under the same conditions, and then the capacity of the batteries were measured and shown in FIG. 4. The unit “C” which is used herein refers to a charging/discharging current rate (C-rate) of a battery, represented by ampere (A), and is usually expressed by a percentage of the battery capacity. Accordingly, “1 C” of the batteries prepared above means a current of 5.5 mA.

Life Characteristics at a High Temperature (4)

Lithium secondary batteries (Capacity: 5.5 mAh) prepared in Examples 2-1, 2-10 and 2-11 and Comparative Examples 2-5 and 2-6 were charged at 60° C. with a constant current of 1.0 C up to 4.35 VA and then with a constant voltage of 4.35 V, and the charging process was completed when the charging current reached 0.275 mA. After left for 10 minutes, the batteries were discharged with a constant current of 1.0 C up to 3.0 V. The charging/discharging was repeated 100 times under the same conditions, and then the capacity of the batteries were measured and shown in FIG. 5. The unit “C” which is used herein refers to a charging/discharging current rate (C-rate) of a battery, represented by ampere (A), and is usually expressed by a percentage of the battery capacity. Accordingly, “1 C” of the batteries prepared above means a current of 5.5 mA.

As can be seen from FIG. 2, lithium secondary batteries having the electrolyte solution comprising fluoroethylene carbonate together with a pyrimidine-based compound exhibit superior life characteristics at a high temperature, as compared to a lithium secondary battery having the electrolyte solution comprising only fluoroethylene carbonate.

Also, as can be seen from FIG. 3, when ethylene sulfite is used as an additive together with fluoroethylene carbonate and a pyrimidine-based compound, lithium secondary batteries exhibit even more superior life characteristics.

As can be seen from FIG. 4, among nitrogen-containing compounds, a pyrimidine-based compound has a superior capability of improving life characteristics, which is deemed to result from the arrangement of nitrogen atoms in suitable positions within the pyrimidine-based compound, and the appropriate inhibition of its reactivity with other additives.

As can be seen from FIG. 5, when the amount of fluoroethylene carbonate and a pyrimidine-based compound satisfy a prescribed range (Examples 2-1, 2-10 and 2-11), even more superior life characteristics are exhibited, than compared to when fluoroethylene carbonate is used in an excessive amount or a pyrimidine-based compound is not used (Comparative Examples 2-5 and 2-6). 

What is claimed is:
 1. A lithium secondary battery comprising: an electrode assembly comprising a cathode, an anode and a separator interposed therebetween; a non-aqueous electrolyte solution for a lithium secondary battery; and a polymerizable film on the surface of the anode, wherein an anode active material of the anode is a carbon-based material, wherein the non-aqueous electrolyte solution consists of an electrolyte salt, an organic solvent, a sulfur-containing compound, and a pyrimidine-based compound selected from the group consisting of pyrimidine, 2-methyl pyrimidine, 4-methyl pyrimidine, 5-methyl pyrimidine, 4,6-dimethyl pyrimidine, and 4,6-dimethoxy pyrimidine, wherein the organic solvent consists of fluoroethylene carbonate and at least one selected from the group consisting of a linear carbonate, a cyclic carbonate, and an ester, wherein the linear carbonate is selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate, wherein the cyclic carbonate compound is selected from the group consisting of ethylene carbonate, propylene carbonate, vinylene carbonate, and vinyl ethylene carbonate, wherein the ester is selected from the group consisting of ethyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone, wherein the amount of the fluoroethylene carbonate is in the range of 10 to 30 vol % based on the total volume of the organic solvent, wherein the amount of the pyrimidine-based compound is in the range of 1 to 10 parts by weight based on 100 parts by weight of the organic solvent, wherein the sulfur-containing compound is 1,3 propane sultone and present in an amount of 2 to 3 parts by weight based on 100 parts by weight of the organic solvent, and wherein the pyrimidine-based compound reacts with the anode to form the polymerizable film on the surface of the anode.
 2. The lithium secondary battery of claim 1, wherein the pyrimidine-based compound is selected from the group consisting of 5-methyl pyrimidine, 4,6-dimethyl pyrimidine, and 4,6-dimethoxy pyrimidine.
 3. The lithium secondary battery according to claim 1, wherein the electrolyte salt is a lithium salt.
 4. The lithium secondary battery according to claim 3, wherein the lithium salt has an anion selected from the group consisting of F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)6P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)2N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻ and (CF₃CF₂SO₂)₂N⁻.
 5. The lithium secondary battery according to claim 1, wherein the cathode has a cathode layer comprising a lithium-containing oxide.
 6. The lithium secondary battery according to claim 5, wherein the lithium-containing oxide is a lithium-containing transition metal oxide.
 7. The lithium secondary battery according to claim 6, wherein the lithium-containing transition metal oxide is selected from the group consisting of LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, Li(Ni_(a)Co_(b)Mn_(c))O₂(0<a<1, 0<b<1, 0<c<1, a+b+c=1), LiNi_(1-y)Co_(y)O₂, LiCo_(1-y)Mn_(y)O₂, LiNi_(1-y)Mn_(y)O₂(0≦y<1), Li(Ni_(a)Co_(b)Mn_(c))O₄(0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn_(2-z)Ni_(z)O₄, LiMn_(2-z)Co_(z)O₄(0<z<2), LiCoPO₄, LiFePO₄ and a mixture thereof. 